Vietnam’s e-visa is applied for online, before travel, through the national portal run by the Immigration Department.
Two things catch people out, and both are avoidable.
The border gate list is the real constraint
An e-visa is not valid at every crossing. It can only be used to enter and exit through border gates on the list approved by the Government of Vietnam.
That list covers land gates such as Bo Y, Cha Lo, Cau Treo, Huu Nghi and Ha Tien, air gates including Noi Bai, Da Nang, Cam Ranh, Cat Bi and Can Tho, and a long list of sea ports including Chan May, Cam Pha, Da Nang and Duong Dong.
The full lists are on the official portal, and they are worth checking against your actual itinerary rather than assuming a smaller airport or an overland crossing is included.
Apply in English, and apply honestly
The portal requires the application to be declared in English, sufficiently and accurately.
It also states that an application will be denied if the Immigration Authority finds any dishonest information in it. Guessing at a detail you are unsure of is a worse strategy than checking it.
Two smaller details
The e-visa is issued to foreigners who are currently abroad and wish to enter Vietnam. It is not the route for someone already in the country on temporary residence.
Declaration and processing times run on the UTC+07:00 time zone, which matters if you are applying from somewhere well ahead of or behind Vietnam and counting days.